Carbon materials for EDLC electrodes have conventionally been manufactured by subjecting a raw material carbon (pitch) to alkaline activation, either directly or after carbonization (dry distillation).
The method of direct activation of a raw material pitch has a problem of providing EDLCs having a capacitance as low as 20 F/cc or so. In contrast, it is known that the method of activation after dry distillation provides high capacitance.
For example, JP-A-2002-25867 (claim 8) discloses a process for manufacturing nonporous carbon that contains graphite-like microcrystalline carbon, and has a specific surface of 270 m2/g or less, and a distance d002 between microcrystalline carbon layers of 0.360 to 0.380 by dry distilling easily graphitizable carbon that is rich in layered graphitic microcrystals at 700° C. to 850° C. to obtain calcined carbon, treating the resulting calcined carbon with caustic alkali at 800° C. to 900° C., and subsequently removing the remaining alkali. EDLCs from carbon electrodes obtained by this process exhibit high capacitances of 29 F/cc or more. Specifically, a carbon material is used which is produced by heating (calcination) a material such as petroleum needle coke or hardened pitch at 650° C. to 850° C. under nitrogen flow for 2 to 4 hours, followed by activation.
In order to manufacture activated carbon for EDLC electrodes by hardening mesophase pitch followed by carbonization and alkaline activation, JP-A-2001-52972 proposed a process of milling bulk mesophase pitch, hardening the pitch followed by carbonization and alkaline activation. JP-A-. 2002-93667 also proposed a process of subjecting a raw material pitch that has a softening point of 150° C. to 350° C., an H/C of 0.5 to 0.9, and an optical anisotropy content of 50% or more to milling into particles having an average particle diameter of 5 to 90 μm, and subsequently to activation at 400° C. to 900° C. These processes provide carbon materials for electric double layer capacitor electrodes having high capacitances of 30 F/cc or more. The methods, however, have a drawback of providing low reproducibility of such capacitances, and thus use of the methods cannot provide high capacitances with stability. In order to overcome the drawback, JP-A-2004-182504 proposed a process of subjecting a pitch having a crystallite thickness Lc(002) of 5.0 nm or more in the c axis direction determined by X-ray diffraction to carbonization and then activation. It is proposed that a raw material pitch having such a property is preferably a synthetic pitch, and can be obtained by polymerizing a condensed polycyclic hydrocarbon having at least one alkyl substituent as a raw material in the presence of hydrogen fluoride and boron trifluoride at 100° C. to 400° C.; and 5 wt % or more of the synthetic pitch is added to an inexpensive pitch without an alkyl substituent such as naphthalene pitch or anthracene pitch.
JP-A-2003-51430 discloses a method of subjecting a raw material carbon containing microcrystalline carbon having a graphite-like crystalline layered structure to heating at 600° C. to 900° C. and subsequently to activation. Use of the method provides EDLCs having capacitances of 30 F/cc or more by using as the raw material carbon a raw carbon composition having an interlayer distance d002 of 0.343 nm or less as determined by X-ray diffraction, and having the crystallite size of microcrystalline carbon of 3.0 nm as determined by X-ray diffraction.